1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly relates to a method and apparatus for image forming capable of preventing image quality deterioration induced by a heat from an optical deflector by insulating f-theta lenses from the optical deflector.
2. Discussion of the Background
A conventional color image forming apparatus includes a copying machine, a facsimile machine, a printing machine and other similar image forming apparatus. In a conventional color image forming apparatus having a structure for a tandem operation, an optical writing apparatus includes a plurality of light sources to emit respective laser beams according to image data. The laser beams irradiate respective image bearing members which are arranged in parallel, so that electrostatic latent images are formed on surfaces of the respective image bearing members.
The conventional color image forming apparatus also includes developing units for operating with the respective image bearing members. The developing units visualize the respective electrostatic latent images formed on the surfaces of the respective image bearing members as toner images of different colors, such as yellow, magenta, cyan and black toner images. In synchronization with movements in the operations as described above, a transfer member such as a transfer sheet is conveyed on a transfer belt. The toner images of different colors are transferred onto the transfer member as an overlaid toner image. The overlaid toner image is fixed by a fixing unit and then is discharged to a sheet discharging part.
The conventional color image forming apparatus generally includes a plurality of optical components separately provided to the optical writing apparatus corresponding to the number of the image bearing members. The plurality of optical components include an optical deflector having a polygon mirror and a motor for driving the polygon mirror. Such optical components are relatively expensive.
When the plurality of optical components are separately provided as described above, part cost and production cost increase, and a large space is required in the optical writing system for arranging the plurality of optical components therein. Therefore, the conventional color image forming apparatus may become large in size.
Under the above-described circumstances, a technique has been proposed as illustrated in FIG. 1.
FIG. 1 shows a conventional color laser printer 1. The conventional color laser printer 1 includes an optical writing apparatus 2, a sheet feeding system 3, an intermediate transfer belt 4, four image bearing members 5A, 5B, 5C and 5D, four primary transfer rollers 6A, 6B, 6C and 6D, registration rollers 7, a secondary transfer roller 8 and a fixing unit 9. The four image bearing members 5A, 5B, 5C and 5D are included in respective image forming units (not shown).
The optical writing apparatus 2 is located between the sheet feeding system 3 and the image forming units in the background color laser printer 1 of FIG. 1.
The sheet feeding system 3 is arranged at a bottom location of the background color laser printer 1.
The intermediate transfer belt 4 is arranged above the image forming units. The intermediate transfer belt 4 forms an endless belt extended with pressure by rollers 10 and 11. The intermediate transfer belt 4 is held in contact between the primary transfer rollers 6A, 6B, 6C and 6D arranged at a position opposite to the image bearing members 5A, 5B, 5C and 5D, respectively, such that the toner images formed on the respective image bearing members 5A, 5B, 5C and 5D are transferred onto the intermediate transfer belt 4 to overlay different color toner images to obtain a recorded image.
The image bearing members 5A, 5B, 5C and 5D are included in the respective image forming units which are arranged in parallel under the intermediate transfer belt 4. Each of the image forming units further includes a charging unit, a developing unit, a discharging unit and a cleaning unit which are not shown in FIG. 1. The image forming units have identical structures with developers of different colors of yellow, magenta, cyan and black.
The color laser printer 1 produces a full-color image through the following operations.
The image bearing members 5A, 5B, 5C and 5D rotate clockwise, which is a direction A in FIG. 1, by a motor (not shown). The charging unit is applied with a charged voltage and then uniformly charges the image bearing members 5A, 5B, 5C and 5D to a predetermined polarity.
The optical writing apparatus 2 optically modulates laser beams L1, L2, L3 and L4 and emits the laser beams L1, L2, L3 and L4 from under the respective image forming units toward the respective image bearing members 5A, 5B, 5C and 5D, respectively, through gaps between the charging units and the developing units of the respective image forming units. The laser beams L1, L2, L3 and L4 irradiate the surfaces of the respective image bearing members 5A, 5B, 5C and 5D so that respective electrostatic latent images are formed on the surfaces of the image bearing members 5A, 5B, 5C and 5D. The electrostatic latent images are generated based on respective single color image data of yellow, magenta, cyan and black which are converted from color image data of a full-color image. The electrostatic latent images are visualized by the respective developing units having developers of different colors corresponding to respective electrostatic latent images as color toner images.
As shown in FIG. 1, the intermediate transfer belt 4 rotates in the direction A. The color toner images formed on the surfaces of the respective image bearing members 5A, 5B, 5C and 5D are sequentially overlaid on the surface of the intermediate transfer belt 4 so that an overlaid color toner image is formed on a surface of the intermediate transfer belt 4.
After the color toner images on the surfaces of the respective image bearing members 5A, 5B, 5C and 5D are transferred onto the intermediate transfer belt 4, the cleaning units corresponding to the image forming units scrapes the surfaces of the respective image bearing members 5A, 5B, 5C and 5D to remove residual toner adhering to the surfaces of the respective image bearing members 5A, 5B, 5C and 5D.
After the cleaning units remove the residual toner, the discharging units corresponding to the image forming units discharge the surfaces of the respective image bearing members 5A, 5B, 5C and 5D so that the image forming units are prepared for the next image forming operations.
The sheet feeding system 3 of FIG. 1 includes a transfer sheet (not shown) in a sheet feeding cassette (not shown). The transfer sheet is fed from the sheet feeding cassette and is conveyed to the registration rollers 7. The registration rollers 7 stop and feed the transfer sheet in synchronization with a movement of the overlaid color toner image towards a transfer area formed between the intermediate transfer belt 4 and the secondary transfer roller 8. The secondary transfer roller 8 is applied with an adequate predetermined transfer voltage having a polarity opposite to the overlaid color toner image on the intermediate transfer belt 4. Thus, the overlaid color toner image is transferred onto the transfer sheet.
The transfer sheet that has the overlaid color toner image thereon is conveyed further upward and passes the fixing unit 9. The fixing unit 9 fixes the overlaid color toner image to the transfer sheet by applying heat and pressure. After the transfer sheet passes the fixing unit 9, the transfer sheet is discharged to a sheet discharging part 12 provided at the upper location of the background color laser printer 1. A belt cleaning unit (not shown) scrapes the surface of the intermediate transfer belt 4 and removes residual toner adhering onto the surface of the intermediate transfer belt 4.
Referring to FIG. 2, a structure of the optical writing apparatus 2 included in the background color laser printer 1 is described.
In FIG. 2, the optical writing apparatus 2 includes four light source units (not shown), a polygon mirror wheel 62 and associated optical components. The optical writing apparatus 2 is encased by an optical housing 50.
The four light source units emit the laser beams L1, L2, L3 and L4 towards the polygon mirror wheel 62.
The polygon mirror wheel 62 is an optical deflector having a double-stage structure including mirrors 62A and 62B. The polygon mirror wheel 62 distributes the laser beams L1, L2, L3 and L4 emitted from the respective light source units symmetrically in two directions for deflecting and scanning the laser beams L1, L2, L3 and L4.
The optical components include two f-theta lenses 63 and 64, imaging lenses 69, 70, 71 and 72, which are also referred to as toroidal lenses, first deflecting mirrors 65, 66, 67 and 68, second deflecting mirrors 73, 74, 75 and 76, and third deflecting mirrors 77, 78, 79 and 80. Each of the f-theta lenses 63 and 64 has a vertical double-layer structure having an upper layer and a lower layer. The optical components direct the laser beams L1, L2, L3 and L4 arranged symmetrically in the above-described two directions with respect to the polygon mirror wheel 62 and direct the laser beams L1, L2, L3 and L4 deflected onto the surfaces of the respective image bearing members 5A, 5B, 5C and 5D so as to form the respective electrostatic latent images thereon.
The optical housing 50 has a flat-box-shaped structure which is hermetically closed, and includes a base plate 50A and a side plate 50B. The base plate 50A is located to a bottom portion of the optical housing 50 and mounts the optical components thereon. The side plate 50B has a shape of a frame surrounding a circumference of the base plate 50A, which forms a tray-like shape. The polygon mirror wheel 62 is disposed in an approximately central portion of the base plate 50A of the optical housing 50, and the optical components as previously described are disposed in the optical housing 50. A top cover 87 is provided at a top portion of the optical housing 50. The top cover 87 has four openings provided with dust-proof glasses 81, 82, 83 and 84 for passing the respective laser beams L1, L2, L3 and L4.
The optical writing apparatus 2 performs image writing through the following operations.
An image is input to a document reading apparatus, such as a scanner (not shown), or an image data output system, such as a personal computer, a word processor and a receiving portion of a facsimile machine, is separated into different color image data. The different color image data is converted into respective color image signals for driving the respective light source units. After the conversion of the color image signals, light sources in the respective light source units, such as semiconductor lasers (LD), are driven to emit the laser beams L1, L2, L3 and L4. The laser beams L1, L2, L3 and L4 emitted from the respective light source units pass through cylindrical lenses (not shown) for correcting an optical face tangle error and reach the polygon mirror 62 directly or after reflected by mirrors (not shown). The laser beams L1, L2, L3 and L4 are deflected in the symmetrical directions by the polygon mirror wheel 62 with the mirrors 62A and 62B which are rotated by at a uniform velocity by an electrical motor (not shown).
As described above, the polygon mirror wheel 62 of FIG. 2 includes the mirrors 62A and 62B having the double-stage structure in which the mirrors 62A and 62B are placed as layers. The mirror 62A deflects the laser beams L1 and L4 and the mirror 62B deflects the laser beams L2 and L3. As an alternative, a single-stage axially longer polygon mirror may achieve a same performance as the double-stage polygon mirror.
After being deflected by the mirrors 62A and 62B of the polygon mirror wheel 62 in two directions, the light beams L1 and L2 pass through the f-theta lens 63 and the light beams L3 and L4 pass through the f-theta lens 64. The light beam L1 deflected by the mirror 62A of the polygon mirror wheel 62 passes through the upper layer of the f-theta lens 63 and the imaging lens 69, is reflected by the first mirror 62, the second mirror 73, the third mirror 77 and then passes through the dust-proof glass 81 to irradiate the image bearing member 5A. The light beam L2 deflected by the mirror 62B passes through the lower layer of the f-theta lens 63, is reflected by the first mirror 66, passes through the imaging lens 70, is reflected by the second mirror 74 and the third mirror 78, and then passes through the dust-proof glass 82 to irradiate the image bearing member 5B. The light beam L3 deflected by the mirror 62B passes through the lower layer of the f-theta lens 64, is reflected by the first mirror 67, passes through the imaging lens 71, is reflected by the second mirror 75 and the third mirror 79, and then passes through the dust-proof glass 83 to the image bearing member 5C. The light beam L4 deflected by the mirror 62A of the polygon mirror wheel 62 passes through the upper layer of the f-theta lens 64 and the imaging lens 72, is reflected by the first mirror 68, the second mirror 76, the third mirror 80 and then passes through the dust-proof glass 84 to irradiate the image bearing member 5D.
In an image forming apparatus including the optical writing apparatus 2 as described above, the polygon mirror wheel 62 is arranged in a vicinity of the f-theta lenses 63 and 64. In a case where the f-theta lenses 63 and 64 are formed by a resin material, heat generated by rotation of the polygon mirror wheel 62 may deteriorate optical characteristics of the f-theta lenses 63 and 64. The deterioration of the f-theta lenses 63 and 64 may erroneously vary speeds of writing images on the surfaces of the respective image bearing members. As a result, scales of the respective images may change. This is mainly caused by heat produced by the rotation of the polygon motor which drives the polygon mirror wheel 62. When the motor of the polygon mirror wheel 62 is controlled or rotated, heat is generated. The heat flows in an air stream generated by rotation of the polygon mirror wheel 62 to hit against the f-theta lenses 63 and 64. At this time, one of the f-theta lenses 63 and 64 is disposed to a position closer than the other to a heat source located upstream of heated air and the f-theta lenses 63 and 64 symmetrically disposed to two different directions are applied with different temperatures of the air stream. Therefore, the temperature change of the f-theta lenses 63 and 64 are not identical. When the f-theta lenses 63 and 64 are warmed up, their optical characteristics may change and the f-theta lenses 63 and 64 may have different copy scales in the main scanning direction due to thermal expansion. Thus, a writing scale may change and a position of writing an image may also change because of heat generated by the polygon mirror wheel 62 and its electrical motor, resulting in deterioration in image quality.